The present invention relates to a hydropneumatic accumulator with a bellows inside the accumulator housing separating a gas space from an oil space. Especially, a metal bellows is fastened to the accumulator housing at one of its ends so that the oil space adjoins the interior of the bellows. The bellows is closed at its other, free end by a closing element movable in response to changes in volume in the gas space and oil space. A valve allows or prevents flow of hydraulic fluid from and into the oil space. The valve, in the event of movement of the closing element corresponding to an increase in the volume of the gas space exceeding an assigned maximum value, may be transferred to its closing position. The closing element is in the form of a trough, the trough being connected by its edge situated on the open end to the associated free end of the bellows. The side wall of the trough extends along the inside of the bellows. The bottom of the trough, as a movable valve element, forms the valve controlling the flow of hydraulic fluid.
Care must be taken to prevent overloading of the bellows in the case of bellows accumulators with rubber or metal bellows. In a conventional accumulator with a metal bellows (see WO 97/46823), for preventing overloading the bellows, a valve lifter of the valve connected to the oil space is mounted in a position relative to the closing element of the metal bellows such that the closing element, in the form of a flat end plate of the metal bellows, strikes the valve lifter when a desired end position has been reached and displaces it into the locked position of the valve so that escape of hydraulic fluid from the oil space is prevented when this end position of the end plate of the metal bellows is reached. Consequently, when the valve is closed, a pressure is maintained in the oil space of the accumulator, which pressure corresponds to the gas pressure currently present in the gas space, even if the connected hydraulic system is not under pressure, so that pressure equilibrium prevails on both sides of the metal bellows.
Although overloading of the bellows is thereby prevented when in operation of the accumulator, the pressure of the hydraulic system connected thereto on the oil side drops. The danger nevertheless continues to exist of damage to the bellows under conditions characterized by excess pressure prevailing on the oil side or by absence of prefill pressure on the gas side. The maximum pressure of the gas space volume more or less corresponds to the stroke volume in the case of the conventional accumulator of the type in question. This volume is determined by the movement of the end plate occurring during contraction and expansion of the metal bellows. The length selected for the stroke, which the end plate can execute inside the accumulator housing, must be of adequate length if a volume of the gas space sufficient for operation of the accumulator is to be made available. Consequently, if gas prefill pressure is absent or excess pressure prevails on the oil side, the prevailing pressure gradient acts on the metal bellows which is fully expanded and accordingly subjected to the greatest mechanical stress. Metal bellows which are either thicker or consist of multiple layers must then be used. The resulting disadvantage is sharp increase in rigidity of the spring leading to a relatively poor response characteristic in operation. Multilayer bellows result in increased weight and higher costs. In addition, a smaller stroke is obtained for each turn of the bellows.
GB-A-1 047 983 discloses a generic hydropneumatic accumulator with a bellows inside the accumulator separating a gas space from an oil space. One end of the bellows is fastened to the accumulator housing so that the oil space adjoins the inside of the bellows. The other, free end of the bellows is closed by a closing element movable in response to appropriate volume changes in the gas space and oil space. The accumulator also has a valve releasing or blocking flow of hydraulic fluid from or into the oil space. The valve may be transferred to its blocking position in the event of movement of the closing element corresponding to an increase in the volume of the gas space exceeding an assigned value. The trough is connected by its edge situated on its open end to the associated free end of the bellows. The side wall of the trough extends along the inside of the bellows. The bottom of the trough is in the form of a movable valve element of the valve controlling the flow of hydraulic fluid.
This conventional solution cannot control the end position of the trough corresponding to the minimum value of the volume of the gas space by means of a valve on the oil side.
Objects of the present invention are to provide an accumulator with an oil/gas bellows separator characterized by improved operating characteristics, in particular also in minimum volume areas of the gas space of the accumulator.
The foregoing objects are obtained by a hydropneumatic accumulator having a valve lifter fastened to the bottom of the trough. The valve lifter extends from the accumulator housing concentrically with the longitudinal axis and is connected to a second movable valve element. On movement of the trough exceeding an assigned minimum gas space volume, this second movable valve element acts in conjunction with a second valve seat blocking flow of hydraulic fluid into the oil space. An advantageous option is created of controlling the end position of the trough, corresponding to the minimum volume of the gas space, by a valve on the oil side. In the event of movement of the trough caused by overpressure on the oil side, inflow of hydraulic fluid into the oil space is blocked by movement of the trough by the valve lifter connected to the trough, and pressure equilibrium is established on the bellows.
During operation of the accumulator of the present invention, once pressure equilibrium has been established between gas prefill pressure in the gas space and hydraulic pressure in the oil space and the trough forming the element closing the bellows is located between its assigned end positions, both valve elements have been lifted from their associated valve seats. Accordingly, flow of fluid has not been blocked. The state of pressure equilibrium at the bellows is continued, irrespective of whether the hydraulic pressure of the oil side may drop in relation to the gas prefill pressure (something which results in closing of the valve) or irrespective of how high the hydraulic pressure may rise relative to the gas prefill pressure (something which in turn results in blocking of the valve, so that additional inflow into the oil space is no longer possible).
The structure employed, in which the bellows encloses the exterior of a trough open on the gas side, is advantageous in several respects. Since the whole interior of the trough is available as part of the gas space, an optimal relationship of total size of the accumulator housing to volume of the gas space is achieved. In the case of the conventional pressure accumulator, an adequate gas space volume can be realized only by allowing an adequate path of travel for the end plate of the metal bellows in the accumulator housing, resulting the overloading problems discussed. In the case of the present invention, the path of travel corresponding to extension of the bellows may be virtually as small as desired. In other words, the accumulator housing may be designed so that beyond a short path of movement of the trough, it forms a mechanical stop, since the whole interior of the trough is available as gas space volume.
In this way, the bellows is protected from extending too far, since it surrounds the exterior of the trough. Under the overpressure prevailing in the gas space, the bellows is also supported mechanically over its entire length by the exterior of the trough. This, in turn, permits build-up of the gas prefill pressure before commencement of operation of the accumulator, that is, before the hydraulic system is connected on the oil side. If the latter is the case and once the operating pressure has been built up in the oil space, pressure equilibrium is maintained on the bellows by the valve controlling flow of hydraulic fluid from the oil space. Since the trough is surrounded externally by the bellows, a very slight xe2x80x9cdead volumexe2x80x9d results between trough and bellows, so that only a small amount of hydraulic fluid need be introduced before the gas space is charged, this in turn resulting in saving of weight and costs.
Since the bottom of the trough is simultaneously in the form of a movable valve element, an especially compact and simple structure is also obtained.
The trough is preferably in the form of a regular or right circular cylinder. The depth can be assigned to the trough such that its bottom serving as valve element acts as a movable round valve disk operating in conjunction with a valve seat formed on the interior wall of the accumulator housing. This arrangement results in a particularly simple valve structure. Since the entire surface of the bottom of the trough is available as a valve disk, an especially large sealing surface can be obtained when the valve is closed. For example, for this purpose an O-ring may be provided for formation of an annular sealing surface in the radial external marginal area.
The valve lifter also performs a control function during the lifting movement of the bellows. Proper seating of the trough bottom serving as a valve element on valve seat and accordingly blocking of the valve are thereby ensured.
Since the bellows operates in the state of equilibrium, a light bellows structure may be employed. This results both in weight reduction and in especially good response behavior.
Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.